The present invention relates to the field of oscillator circuits and more particularly to oscillator circuits that are fabricated by using integrated circuit processes. The subject invention is a tunable oscillator that utilizes an input signal having a fixed frequency such as a crystal oscillator. The invention is useful in applications that require a low cost oscillator having an accurate and tunable output frequency.
In general, an oscillator is a circuit that converts a power supply voltage into a time dependent voltage or current waveform having frequency. Oscillators have a wide range of applications. Often oscillators are used to provide a timing reference in digital systems such as digital computers and related equipment. To obtain high performance, which includes high frequency and low error rate, the oscillator must be stable and precise. Oscillators are also used in communication systems to provide a carrier signal that is to be modulated for transmission. The carrier signal must have a stable and accurate frequency to prevent interference with other channels. Oscillators are also used in electronic instrumentation, such as signal generators, spectrum analysers and automated test equipment (ATE). Often the accuracy of the instrument is a direct function of the precision of the primary source of frequency, which is the oscillator.
Often a crystal oscillator circuit is used for applications that require frequency to be accurate within 1 per cent. The frequency tolerance of the crystal oscillator is much less than 0.1 per cent. As such crystal oscillators are well suited for many applications. However, there are many applications in which crystal oscillators are not well suited. The crystal oscillator is not well suited for most applications that require the oscillator frequency to be tuned or varied over a significant frequency range.
FIG. 1 discloses a typical prior art CMOS crystal oscillator, that includes CMOS inverter 150 and feedback resistor 130 on an integrated circuit 100, that are connected to a quartz crystal 140 and capacitors 110 and 120. CMOS inverter 150 amplifies the input voltage of the CMOS inverter. The CMOS inverter 150 provides a phase shift of 180 degrees by inverting the input signal. The output impedance of the CMOS inverter 150 and the first capacitor 110 produce a pole that provides additional phase shift that approaches 90 degrees. Quartz crystal 140 and the second capacitor 120 provides additional phase shift. As such, oscillator 100 of FIG. 1 oscillates at a frequency where the total phase shift is 360 degrees. The feedback resistor 130 provides a conductive path from the output to the input, so that CMOS inverter 140 is self biased in the high gain region.
FIG. 1 also includes CMOS inverters 160, 162 and 170. The CMOS inverter 162 provides a buffered output signal to drive an external load. The output of inverter 162 is used as an external clock signal to operate other devices at the same frequency used as the internal signal. Inverter 162 is often required to prevent degradation of the internal clock signal at the input of the CMOS inverter 150. CMOS inverters 160 and 170 provide additional amplification and buffering of the clock signal that drives the internal CMOS logic within the integrated circuit.
Quartz is piezoelectric. With proper electrical stimulation the quartz crystal 140 produces mechanical vibrations. These mechanical vibrations are then converted back to an electrical current and voltage. Over a narrow range of frequency, the mechanical vibrations occur with very small frictional losses. As such a quartz crystal requires very small power to sustain oscillations. As a consequence, the quartz crystal has very high Q and a very narrow bandwidth of operation.
FIG. 2 shows the equivalent electrical circuit of the quartz crystal 140. Quartz crystal 140 includes the elements Cp 210, Ls1 212, Rs1 214, and Cs1 218 which represent the fundamental mode. In addition quartz crystal 140 includes the elements Ls3 232, Rs3 234 and Cs3 238 representing the third overtone. Also, quartz crystal 140 includes Ls5 252, Rs5 254, and Cs5 258 which is the model for the fifth overtone, and so on. As such, quartz crystal 140 exhibits resonance at a fundamental mode, and at frequencies near the odd multiples of the fundamental mode frequency.
At low frequency inductors have low reactance and capacitors have high reactance. As the frequency increases the reactance of the inductors increase and the reactance of the capacitors decrease. In series the net reactance is the reactance of the inductors minus the reactance of the capacitors. At series resonance, the reactance of Ls1 and Cs1 are equal and the crystal has an effective resistance equal to Rs1. At a frequency just slightly above series resonance, the quartz crystal 140 exhibits parallel resonance. When parallel resonance occurs, capacitor Cs1 218 is in series with Cp 210. Cp is much larger in value than Cs1. As such the series capacitance of Cp and Cs1 is just slightly smaller than Cs1. So the parallel resonant frequency is just slighty higher than the frequency for series resonance. Any attempt to tune the quartz crystal by adding external capacitance shall cause the series capacitance of Cs1 and Cp to approach the value of Cs1.
As such the frequency of a crystal oscillator is tunable over an extremely small band of frequency. The tuning range is often only several parts per million. A typical crystal oscillator has a tuning range less than 200 Hz. The tuning range is so small that the crystal oscillator is considered "untunable".
There are various prior art techniques used to realize a tunable oscillator using a crystal oscillator reference. One such example is indirect frequency synthesis as described in U.S. Pat. No. 5,521,556 which is hereby incorporated by reference in its entirety. FIG. 3, which is labeled prior art, discloses a digital tuned oscillator (DTO). The DTO 300 uses a binary counter 310 to measure the output frequency of a voltage controlled oscillator (VCO) 320. The output of the binary counter 310 is compared to a binary input 340. If the counter output is less than the binary input 340, then the input voltage to the VCO 320 is adjusted to increase frequency. If the count is higher than the digital input, the input voltage to VCO 320 is adjusted to decrease the VCO frequency. So by changing the input code applied to input 340, the frequency of this circuit is altered.
However, indirect frequency synthesis has certain disadvantages. The circuit topology is very complex, which requires a large amount of chip area at added cost. The circuit is difficult and expensive to simulate. Another disadvantage is that frequency is adjustable only in discrete steps and as such is not continuously variable. Yet even another disadvantage is that increasing the resolution shall significantly increase the circuit complexity, thereby increasing the chip area and cost.
Direct digital synthesis, not shown, uses an input code applied to the input of a digital accumulator to compute states that are multiples of the input word. As such the output of the accumulator generates addresses that are applied to the input of a read only memory (ROM). The output of the ROM are applied to the input of a digital to analog converter (DAC). This arrangement is capable of synthesizing an arbitrary waveform programmed into the ROM.
Direct digital synthesis also has disadvantages. Direct digital synthesis has a high degree of circuit complexity. This technique requires a DAC, a ROM and an accumulator. An increase in resolution also requires a significant increase in circuit complexity. Another disadvantage is that frequency is only adjustable in discrete steps. Yet, another disadvantage is that the highest frequency to be synthesized must be less than half the frequency of the input clock. A higher clock frequency is obtainable by using a phase lock loop (PLL) to increase input frequency. However, the PLL significantly increases the circuit complexity.
In summary, the crystal oscillator produces an output signal having a very accurate and stable frequency. However, the tunable frequency range of the quartz crystal is extremely small. As such the crystal oscillator is unuseable in applications requiring a tunable oscillator. The use of frequency synthesis provides some degree of tuning. However, this tuning occurs only in discrete frequency steps. Increasing resolution provides smaller frequency steps, but then circuit complexity and cost increase significantly. Therefore, a new approach is needed for applications that require precision, low cost and tunability.